Congress last March approved more than $600 million for 33 miles (53 kilometers) of new barriers in the Rio Grande Valley. While President Donald Trump and top Democrats remain in a standoff over Trump’s demand for $5.7 billion in border wall funding, U.S. Customs and Border Protection has pushed ahead with building what’s already funded.

That construction was often described as fencing, and the government funding bill that included construction was supported by some Democrats in the House and Senate. CBP refers to what it plans to build as a “border wall system.”

According to designs it released in September , CBP intends to build 25 miles (40 kilometers) of concrete walls to the height of the existing flood-control levee in Hidalgo County next to the Rio Grande, the river that forms the U.S.-Mexico border in Texas. On top of the concrete walls, CBP will install 18-foot (5.5-meter) steel posts and clear a 150-foot (45-meter) enforcement zone in front.

Maps released by CBP show construction would cut through the butterfly center, a nearby state park, and a century-old Catholic chapel next to the river.

Many landowners oppose a border wall and have vowed to fight the U.S. government if it tries to seize their property through eminent domain. Court fights over condemning land could take months.

CBP said in its statement that it intends to start construction on federally owned land. Environmental advocates expect the government to use land that’s part of the Lower Rio Grande Valley National Wildlife Refuge.